Many techniques have been devised for lubricating trunnions and their associated bushings in gear pumps or motors. Some approaches use high pressure fluid from the outlet side of the pump or motor for such lubrication. Typically, such an approach results in the loss of volumetric and overall efficiency of the pump or motor. Other approaches utilize low pressure fluid from the inlet side of the pump or motor to lubricate the trunnions and bushings. These latter approaches are based upon the principle that the inlet fluid is typically at atmospheric pressure and that the fluid at the point of the unmeshing of the gears adjacent the inlet side is at a pressure that is less then atmospheric pressure. Because of this pressure gradient, the fluid at the inlet side can theoretically pass through the area between the trunnions and the bushings, thus lubricating same. However, in order to accomplish the foregoing, a groove must be provided in the face of the thrust plate to permit the passage of fluid from the bushings to the inlet side of the pump or motor. Because of the configuration and size of the groove, and the positioning of same, it has been found that the foregoing pressure gradient is significantly less than what is theoretically expected, resulting in a substantial loss in volumetric efficiency, inefficient trunnion lubrication and elevated bushing temperature. This is particularly true if the groove for the passage of the fluid from the bushing to the inlet side of the pump is on the surface of the thrust plate adjacent the sides of the gears, as in U.S. Pat. No. 4,470,776. The positioning of the foregoing groove on this particular surface of the thrust plate decreases the effectiveness of the required seal between the thrust plate and the face of the gears which, in turn, diminishes the pressure gradient across the bushing permitting fluid to traverse down the trunnion. Such "leakage" increases volumetric losses and reduces the resulting output flow from the pump or motor.
Because of the foregoing, it has become desirable to develop a lubrication system for a gear pump or motor that provides sufficient lubrication of the trunnions and bushings therein and does not affect the overall efficiency of the device.